| Summary: | 博士 === 國防大學中正理工學院 === 國防科學研究所 === 97 === The purpose of this study was to investigate the phase evolution of the Ni, Al, and Ti powder mixture with a composition of Ni60.1Al27.2Ti12.7 during both self-propagating high temperature (SHS) and mechanical alloying (MA) processes. The DSC thermal analysis of this powder mixture, from 20 to 1200℃, shows that there are two exothermic and two endothermic reactions in the SHS process. The phases present in heat-treated powder and MA samples were then identified using X-ray diffraction and examined metallographically. Data obtained from thermal analysis, XRD experiments and microstructure examination may be interpreted in terms of the Ni–Al binary and Ni–Al–Ti ternary phase diagrams. The growth of the γ phase at the expense of the β phase was analyzed kinetically and shows that grain boundary diffusion is responsible for the γ growth in samples heat-treated at lower temperatures (670 or 875℃), while volume diffusion becomes dominant for the higher heat-treatment temperature (1165℃) in the SHS process. The sequence of formation of stable intermetallic phases was β–Ni(Al,Ti), γ–Ni3(Al,Ti) and β–Ni2AlTi when annealed from room temperature to 1200℃, and the sequence of formation of phases was the same as that in the MA process.
In addition, disordered Ni75Al25-xTix (x = 0, 4, 8, 12) alloys were also prepared from elemental powders using a high-energy ball milling process. We analyzed the as-milled powders with an X-Ray Diffraction diffractometer (XRD) and a differential scanning calorimeter (DSC). It is found that the thermal stability of the disordered structure of Ni3(Al,Ti) of these powders increased with Ti content. The effective activation energy for ordering of Ni3(Al,Ti) was evaluated according to Kissinger’s plot. The results show that the effective activation energy of ordering and the rate of ordering transformation both decrease with Ti content.
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